The present invention relates to powder coating compositions with improved adhesion and filiform corrosion resistance.
More specifically, the invention relates to a powder coating composition comprising a solid particulate film-forming mixture of a polymer containing reactive functional groups and a curing agent therefor having functional groups reactive with the functional groups of the polymer with an additive to improve adhesion and filiform corrosion resistance.
Powder coating compositions for use in painting are extremely desirable. Such coating compositions can eliminate the organic solvents used in liquid paints. When the powder coating composition is thermally cured, little, if any, volatile material is given off to the surrounding environment. This is a significant advantage over liquid paints in which organic solvent is volatilized into the surrounding atmosphere when the paint is cured by heating.
A particular problem which often results from the use of powder coatings, particularly over aluminum substrates, is filiform corrosion. Filiform corrosion generally occurs in wet environments at the site of a surface defect in the presence of soluble ionic species. As described in Filiform Corrosion in Polymer-coated Metals, A. Bautista, PROGRESS IN ORGANIC COATINGS 28 at pages 49-58 (1996), this deterioration process gives rise to corrosion products which are characterized by a filamentous, worm-like appearance under the coatings. The "filaments" typically exhibit an arborescent structure and grow directionally under the coating.
Filiform corrosion can result in delamination of the coating from a metal substrate and it has become a matter for increasing concern in the areas of automotive, industrial and architectural coatings. Accordingly, it is desirable to provide a powder coating composition with improved filiform corrosion. It has been found that incorporation of certain polyesters having multiple carboxylic acid functional groups improves the filiform corrosion resistance of the powder coating composition.
Pending U.S. patent application Ser. No. 08/995,790 filed Dec. 22, 1997 discloses a powder coating composition having improved filiform corrosion resistance, wherein the improvement is due to the inclusion in the composition of an organic polysiloxane having various pendant reactive functional groups. Examples of these pendant functional groups include COOH, NCO, carbamate, primary and secondary amine and epoxy functional groups.
U.S. Pat. No. 5,543,464 teaches a powder coating composition comprised of an epoxy functional group containing acrylic polymer, and, as curing agents, a crystalline polycarboxylic acid and a semi-crystalline polycarboxylic acid group containing polyester. The polycarboxylic acid group containing polyesters are based on a polycondensation reaction of (cyclo) aliphatic and/or aromatic polyols with (cyclo) aliphatic and/or aromatic polycarboxylic acids or anhydrides, esters or acid chlorides based on these acids. These acid functional polyester curing agents impart enhanced flexibility and improved impact resistance to the resultant coating.
U.S. Pat. No. 4,937,288 discloses a thermosetting powder coating composition which comprises a co-reactable particulate mixture of a carboxylic acid group-containing acrylic polymer, a second carboxylic acid group-containing material with crystallinity sufficient to assist the flow of the composition as it cures, and a beta-hydroxyalkylamide curing agent. The second carboxylic acid group-containing material is selected from the class of C.sub.4 to C.sub.20 aliphatic dicarboxylic acids, polymeric polyanhydrides, and preferably, low molecular weight carboxylic acid group-containing polyesters. The second carboxylic acid group-containing material provides flexibility and impact resistance of the resultant coating, as well as assisting in flow during cure of the powder coating composition.
U.S. Pat. No. 5,407,707 discloses a powder coating composition which comprises a solid, particulate mixture of 60 to 90 percent by weight of an epoxy functional copolymer and 10 to 40 weight percent of a polycarboxylic acid crosslinking agent. Suitable polycarboxylic acid crosslinking agents include crystalline aliphatic materials such as adipic, succinic, sebacic, azelaic and dodecanedioic acid. Low molecular weight polycarboxylic acid group-containing polyesters and half-acid esters can also be used.